The motor starter can be used to control an electric motor downstream of the motor starter. To this end, an energy supply for the electric motor downstream of the motor starter is supplied via the motor starter so that the electric motor is connected to a supply network. The supply network is in particular a low-voltage network of an industrial installation (for example three phases, 400 volts, 50 hertz).
To control the energy supply of the downstream electric motor, the motor starter comprises a control unit and a first current path. A first phase of the supply network is supplied via the first current path to the downstream electric motor. The first current path comprises a semiconductor switch and an electromechanical switch element, wherein the semiconductor switch and the switch element are connected in series and can be controlled by way of the control unit.
When switching three-phase synchronous motors with semiconductor switches, when the electric motor has been switched off by way of a semiconductor switch, the semiconductor switch is located between two voltage sources so that a blocking voltage is applied to the semiconductor switch. The blocking voltage of the semiconductor switch is the electric voltage, which is applied via the semiconductor switch. One voltage source is the supply network. The other voltage source is the voltage induced by the electric motor, which is present for as long as there is still a magnetic field in the electric motor.
Since, on switching off by the motor starter, the speed of the electric motor is lower than its synchronous speed and the speed dies down after the electric motor is switched off, the voltage generated by the electric motor always has a frequency lower than the frequency of the voltage of the supply network. This results in instants of time for the motor starter in which the voltage difference and hence the voltage stress of the semiconductor switch can increase to the total of the two peak voltages. Therefore, the maximum allowable blocking voltage of the semiconductor switch used has to be set correspondingly high. To date, this has been achieved in that the semiconductor switch of a motor starter must have a maximum allowable blocking voltage, which is higher than the maximum voltage that occurs at the semiconductor switch in respect of an allowable system for the motor starter. The necessary maximum allowable blocking voltage for the semiconductor switch of a motor starter is usually calculated as follows:USperr≥UNetz·√{square root over (2)}·2·T     USperr=Maximum blocking voltage of the semiconductor switch    UNetz=Nominal voltage of the supply network    T=Tolerance factor
At 500 volts nominal supply network voltage with 10% tolerance (T=1.1), the blocking voltage is 1556 volts. Therefore, usually a semiconductor switch with a maximum allowable blocking voltage of 1600 volts would be used inside a motor starter.